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Java in a Nutshell

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Classes and Objects in Java
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3.4 Class Methods

Let's define a new method in our Circle class. This one tests whether a specified point falls within the defined circle:

public class Circle {
    double x, y, r;
    // is point (a,b) inside this circle?  
    public boolean isInside(double a, double b)
    {
        double dx = a - x;
        double dy = b - y;
        double distance = Math.sqrt(dx*dx + dy*dy);
        if (distance < r) return true;
        else return false;
    }
        .
        .  // Constructor and other methods omitted.
        .
}

What's this Math.sqrt() thing? It looks like a method call and, given its name and its context, we can guess that it is computing a square root. But the method calls we've discussed are done through an object. Math isn't the name of an object that we've declared, and there aren't any global objects in Java, so this must be a kind of method call that we haven't seen before.

static Methods

What's going on here is that Math is the name of a class. sqrt() is the name of a class method (or static method) defined in Math. It differs from the instance methods, such as area() in Circle, that we've seen so far.

Class methods are like class variables in a number of ways:

  • Class methods are declared with the static keyword.

  • Class methods are often referred to as "static methods."

  • Class methods are invoked through the class rather than through an instance. (Although within the class they may be invoked by method name alone.)

  • Class methods are the closest Java comes to "global" methods. Because they must be referred to by the class name, there is no danger of name conflicts.

No this

Class methods differ from instance methods in one important way: they are not passed an implicit this reference. Thus, these this-less methods are not associated with any instance of the class and may not refer to any instance variables or invoke instance methods.

Since class methods are not passed a this reference, and are not invoked through an object, they are the closest thing that Java offers to the "normal" C procedures that you may be accustomed to, and may therefore seem familiar and comforting. If you're sick and tired of this object-oriented business, it is perfectly possible to write complete Java programs using only class methods, although this does defeat an important purpose of using the language!

But don't think that class methods are somehow cheating--there are perfectly good reasons to declare a method static. And indeed, there are classes like Math that declare all their methods (and variables) static. Since Math is a collection of functions that operate on floating-point numbers, which are a primitive type, there are no objects involved, and no need for instance methods. System is another class that defines only class methods--it provides a varied collection of system functions for which there is no appropriate object framework.

A Class Method for Circles

Example 3.5 shows two (overloaded) definitions of a method for our Circle class. One is an instance method and one is a class method.

Example 3.5: A Class Method and an Instance Method

public class Circle {
    public double x, y, r;
    // An instance method.  Returns the bigger of two circles.
    public Circle bigger(Circle c) { 
        if (c.r > r) return c; else return this; 
    }
    // A class method.  Returns the bigger of two circles.
    public static Circle bigger(Circle a, Circle b) {
        if (a.r > b.r) return a; else return b;
    }
        .
        .  // Other methods omitted here.
        .
}

You would invoke the instance method like this:

Circle a = new Circle(2.0);
Circle b = new Circle(3.0);
Circle c = a.bigger(b);         // or, b.bigger(a);

And you would invoke the class method like this:

Circle a = new Circle(2.0);
Circle b = new Circle(3.0);
Circle c = Circle.bigger(a,b);

Neither of these is the "correct" way to implement this method. One or the other will seem more natural, depending on circumstances.

A Mystery Explained

Now that we understand class variables, instance variables, class methods, and instance methods, we are in a position to explore that mysterious method call we saw in our very first Java "Hello World" example:

System.out.println("Hello world!");

One hypothesis is that println() is a class method in a class named out, which is in a package named System. Syntactically, this is perfectly reasonable (except perhaps that class names always seem to be capitalized by convention, and out isn't capitalized). But if you look at the API documentation, you'll find that System is not a package name; it is the name of a class (which is in the java.lang package, by the way). Can you figure it out?

Here's the story: System is a class. It has a class variable named out. out refers to an object of type PrintStream. The object System.out has an instance method named println(). Mystery solved!

Static Initializers

Both class and instance variables can have initializers attached to their declarations. For example:

static int num_circles = 0;
float r = 1.0;

Class variables are initialized when the class is first loaded. Instance variables are initialized when an object is created.

Sometimes we need more complex initialization than is possible with these simple variable initializers. For instance variables, there are constructor methods, which are run when a new instance of the class is created. Java also allows you to write an initialization method for class variables. Such a method is called a static initializer.

The syntax of static initializers gets kind of bizarre. Consider that a static initializer is invoked automatically by the system when the class is loaded. Thus there are no meaningful arguments that can be passed to it (unlike the arguments we can pass to a constructor method when creating a new instance). There is also no value to return. So a static initializer has no arguments and no return value. Furthermore, it is not really necessary to give it a name, since the system calls the method automatically for us. What part of a method declaration is left? Just the static keyword and the curly brackets!

Example 3.6 shows a class declaration with a static initializer. Notice that the class contains a regular static variable initializer of the kind we've seen before, and also a static initializer--an arbitrary block of code between { and }.

Example 3.6: A Static Initializer

// We can draw the outline of a circle using trigonometric functions.
// Trigonometry is slow though, so we pre-compute a bunch of values.
public class Circle {
    // Here are our static lookup tables, and their own simple initializers.
    static private double sines[] = new double[1000];
    static private double cosines[] = new double[1000];
    // Here's a static initializer "method" that fills them in.  
    // Notice the lack of any method declaration!
    static {
        double x, delta_x;
        int i;
        delta_x = (Circle.PI/2)/(1000-1);
        for(i = 0, x = 0.0; i < 1000; i++, x += delta_x) {
            sines[i] = Math.sin(x);
            cosines[i] = Math.cos(x);
        }
    }
        .
        .  // The rest of the class omitted.
        .
}

The syntax gets even a little stranger than this. Java allows any number of static initializer blocks of code to appear within a class definition. What the compiler actually does is to internally produce a single class initialization routine that combines all the static variable initializers and all of the static initializer blocks of code, in the order that they appear in the class declaration. This single initialization procedure is run automatically, one time only, when the class is first loaded.

One common use of static initializers is for classes that implement native methods--i.e., methods written in C. The static initializer for such a class should call System.load() or System.loadLibrary() to read in the native library that implements these native methods.

Instance Initializers

In Java 1.1, a class definition may also include instance initializers. These look like static initializers, but without the static keyword. An instance initializer is like a constructor: it runs when an instance of the class is created. We'll see more about instance initializers in Chapter 5, Inner Classes and Other New Language Features, Inner Classes and Other New Language Features.


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